The present invention relates to a structure for preventing the deposition of foreign substance on a photo-mask or a reticle (hereinbelow, referred generally to as the photo-mask for both members) used in a manufacturing step for an integrated circuit. In particular, the present invention relates to a structure for attaching a pellicle suitable for a photo-lithography method using an F2 laser (wavelength: 157.6 nm) to a photo-mask.
In a photo-lithography process used in a manufacturing step for an integrated circuit, the formation of a pattern is carried out by exposing a semiconductor wafer applied with a photoresist material to light. If a photo-mask has a flaw or a foreign substance depositing on it, the trace of the flaw or the foreign substance is transferred to the wafer together with the pattern, whereby short-circuiting or breaking of wire may cause.
Accordingly, there has been used a method for attaching a pellicle to a single or both surfaces of the photo-mask in order to prevent foreign substance from depositing on the surface or surfaces of the photo-mask.
The conventional pellicle was of a type having an outer configuration as shown in a front view and a side view of FIG. 5, which was formed by bonding a pellicle membrane 2 made of nitrocellulose or a fluorocarbon resin having a thickness of from several nm to several xcexcm, to a pellicle frame 1 of aluminum or the like with an adhesive 3. When used, the pellicle was fixed so as to cover the pattern formed on the photo-mask. Such pellicle in which the membrane of organic material was used had the following disadvantage. When light of short wavelength was used to obtain accurate light exposure, an organic component was decomposed due to the irradiation of a strong optical energy. Accordingly, the conventional pellicle did not have a sufficient durability to the number of times of irradiation of energy, in other words, to an operating time, in the actual light exposure,. In consideration of this, there has been studied to use a pellicle sheet formed by processing synthesized silica glass into a thin sheet, instead of the pellicle membrane made of an organic resinous material in recent years. The synthesized silica glass is glass consisting substantially of only silicon oxide which is obtained, for example, by growing porous material composed of silicon oxide, called xe2x80x9csootxe2x80x9d, by reacting a silicon source and an oxygen source in a gaseous phase, followed by sintering. Since the pellicle sheet made of the synthesized silica glass has an excellent transmittance, an anti-reflection membrane is often disposed at its single or both surfaces.
When the pellicle in which the synthesized silica glass is used as the pellicle sheet is actually used in a light exposure step, the synthesized silica glass itself absorbs light energy to generate heat whereby the pellicle sheet expands. Further, the pellicle frame itself receives the heat. Since metal, e.g., aluminum, titanium or the like, which is conventionally used for the material for the pellicle frame has a different thermal expansion coefficient from that of the synthesized silica glass, a stress is generated in the pellicle sheet made of the synthesized silica glass, whereby there may cause a change of light path due to birefringence or a change of light path due to a mechanical flexure. Accordingly, it is desirable that a material for the pellicle frame should have the same or closer thermal expansion coefficient to that of the synthesized silica glass.
On the other hand, when the synthesized silica glass is used for the pellicle sheet, it has been proposed to use a pellicle sheet having a thickness of from 0.01 to 2 mm. However, when a pellicle sheet having such thickness is used in an actual light exposure step and if there is a scattering of thickness in the pellicle sheet, it has been known that exposed light 4 is refracted to change the light path as shown in FIG. 6. The change of light path causes a shift of the position of the pattern to be transferred, and a good lithographic operation can not be conducted. Accordingly, it is proposed that the parallelism of upper and lower planes, which is permissible for the pellicle sheet, is 0.1 xcexcm/50 mm.
Further, even in a case that a pellicle sheet satisfying the above-mentioned parallelism is used, the shift of the position of the pattern to be transferred is caused if the parallelism between two planes: the pellicle sheet 2 and the surface of the photo-mask 5 opposing to the pellicle sheet 2, which are fixed at upper and lower planes of the pellicle frame 1 as shown in FIG. 7, is bad.
The pellicle is constituted by bonding the pellicle sheet to the pellicle frame. If the flatness of the bonded plane is inaccurate, the pellicle sheet after having been bonded is curved because the pellicle sheet follows the shape of an upper plane of the pellicle frame, or creates an optical problem such as an increased refringence due to the inaccurate flatness. Or, if the flatness of a lower plane of the pellicle frame is inaccurate, distortion or flexure is generated in the pellicle frame itself when it is bonded to the photo-mask, and there is a possibility of causing the flexure in the pellicle sheet.
Accordingly, it is required that the flatness of each plane of the pellicle frame (when the entire surface of an upper or lower plane constituting a frame-like member is considered, the flatness indicates the maximum difference of concave-convex in an ideal plane) is not more than 1 xcexcm, and the parallelism between the upper and lower planes (the maximum difference at an optional point in the entire surface of the frame-like member) is not more than 2 xcexcm. Further, it is necessary to take a method for bonding the pellicle sheet to the pellicle frame or the photo-mask to the pellicle frame while the above-mentioned accuracy should be maintained.
As the way of solving the above-mentioned problem, there has been attempted a method for reducing the thickness of an adhesive between the pellicle sheet and the pellicle frame, and an adhesive or a sticking material (hereinbelow, referred simply to as the adhesive for both materials) between the photo-mask and the pellicle frame.
These adhesives are generally composed of an organic material. Accordingly, when light having a large energy is irradiated as described before, the adhesive is decomposed or deteriorated to cause peeling-off or generation of dust at the bonded portions, or gas generated from an organic component reduces the transmittance of light having a usable wavelength.
On the other hand, the inventors of this application proposed, approach to prevent the deterioration of an adhesive between the pellicle sheet and the pellicle frame in Japanese Patent Application No. 2000-302306. According to this approach, it can be expected to obtain a predetermined effect if the above-mentioned flatness of the bonded portion of the pellicle frame is satisfied. However, the above-mentioned invention does not mention a sufficient approach to the bonding between the pellicle frame and the photo-mask.
There is also a problem in the method for obtaining the pellicle frame having a sufficient accuracy of configuration.
When a frame-like member is produced from silica glass, there is considered a method that a rod-like member is fused and treated to form a frame-like member, or a method that silicon oxide powder put in a mold is heated for vitrification to form a frame-like member, then, polishing is carried out for upper and lower planes of the frame-like member.
When the conventional polishing method (i.e. single face polishing) is applied to the manufacture of the pellicle frame, it may be possible to obtain a good flatness for each plane of the pellicle frame. However, in evaluation as to whether or not each plane has been able to be processed to have a predetermined accuracy, the conventional optical measuring method using a Newton ring is insufficient for the analysis for its shape because the frame width of the pellicle frame is narrow as about 1 to 5 mm. An accurate measurement can not be expected unless an expensive device such as a three-dimensional measuring instrument is used.
Further, when a frame-like member made of silica glass is processed with a double face polishing machine in order to obtain an excellent parallelism, there were disadvantages as follows. The pellicle frame has generally outer dimensions of 120xc3x97120 mm or more, a frame width of 1 to 5 mm and a height of 3 to 6 mm. Accordingly, it is weak in strength as a whole. Accordingly, the pellicle frame was broken during the processing. In order to avoid the breakage, it was necessary to conduct a light polishing operation with light load and low revolution for a long time, whereby working efficiency was reduced remarkably.
The inventors of this application have achieved the present invention by finding that the surface of the pellicle frame to be bonded to the photo-mask can be made flat with high accuracy.
In accordance with the present invention, there is provided a structure for attaching a pellicle to a photo-mask wherein the pellicle comprises a pellicle frame and a pellicle sheet attached to an opening portion formed in the pellicle frame, the structure for attaching a pellicle to a photo-mask being characterized in that at least a portion of the pellicle frame surface in contact with the photo-mask has a direct contact with the photo-mask without interposing an adhesive.
Namely, it is considered that the parallelism between the pellicle sheet and the plane of photo-mask can be maintained when they contact with each other in their entire plane area or a part of the plane without interposing an adhesive. As examples of the structure for this purpose, there are following methods: (1) an auxiliary member for pressing the pellicle frame to the photo-mask is used. In particular, an auxiliary member having an L-shape in cross section is made engagement with at least a part of the pellicle, and the auxiliary member is bonded to the photo-mask to press the pellicle frame to the photo-mask, (2) an adhesive is disposed at an outer circumferential portion of the pellicle frame and the pellicle frame is bonded to the photo-mask, and (3) a concave portion is formed by processing the pellicle frame surface to bonded to the photo-mask, and the pellicle frame is bonded to the photo-mask by filling an adhesive in the concave portion.
The adhesive referred to in this text may be a poly butene resin type, acrylic resin type, epoxy type, silicone type, fluorocarbon resin type or the like in the same manner as used in the conventional technique. Further, the adhesive referred to in this text includes a sticking material.
For the material for the pellicle sheet, there is no limitation as far as the material can transmit exposed light. However, it is preferable to use synthesized silica glass because it has a high transparency and durability to ultra-violet rays of short wavelength.
The material for the pellicle frame is desirably such one having the same thermal expansion coefficient as the synthesized silica glass. In this connection, the synthesized silica glass is the most desirable, and fused silica glass, glass having other components, ceramics, an alloy or the like may be used.
Further, the present invention is to provide a structure for attaching the pellicle produced by a method including the following steps to a photo-mask, in order to make the pellicle frame surface to be bonded to the photo-mask flat with high accuracy, and to make the parallelism between upper and lower planes of the pellicle frame parallel with high accuracy;
a preparation step of preparing a silica glass sheet having larger outer dimensions than the outer configuration of the pellicle frame and a larger thickness than the height of the pellicle frame;
a polishing step of polishing both surfaces of the silica glass sheet, and
a processing step of processing the silica glass sheet after the polishing to obtain a frame-like member.